1. Field of the Invention
The invention relates to an adsorption process for purifying a gas mixture rich in hydrogen and carbon monoxide, usually called an H2/CO mixture or syngas, before it is cryogenically treated for the purpose of producing a CO-rich fraction and/or one or more H2/CO mixtures of defined content, such as for example a 50 mol % H2/50 mol % CO mixture, and generally a hydrogen-rich fraction.
2. Related Art
Syngas mixtures may be obtained in several ways, especially:                by steam or CO2 reforming;        by partial oxidation;        by hybrid processes, such as the ATR (AutoThermal Reforming) process, which is a combination of steam reforming and partial oxidation, using gases such as methane or ethane;        by gasification of coal;        or recovered gases, such as waste gases downstream of acetylene manufacturing units.        
In addition to hydrogen and carbon monoxide as main components, many impurities such as carbon dioxide, water or methanol often form part of syngases.
Among purification processes, the TSA (Temperature Swing Adsorption) process is a cyclic process in which each of the adsorbers alternates between adsorption steps, during which the impurities are retained in the adsorbent, and regeneration steps, during which a heating phase is used in particular to extract the impurities from the adsorbent. The typical operating cycle of this type of unit has been described in document WO-A-03/049839.
Units for carrying out TSA purification processes are generally designed so as to obtain a syngas of cryogenic quality, i.e. such that, when said syngas is cooled in the cold box, any deposits of impurities are low enough to ensure satisfactory operation of said cold box for several years, therefore without becoming clogged, without the exchange line suffering thermal deterioration and with no risk to the safety of the equipment.
This is achieved with a maximum residual CO2 content generally of the order of 0.1 ppm and even lower contents, of around 1 ppb, for the other impurities.
To limit intervention on these purification units, they are also designed with initial design margins wide enough to ensure correct operation for several years without having to replace the adsorbents.
Despite all the precautions, it turns out that the lifetime of these units is substantially shorter than initially predicted.
In normal operation, a CO2 analyser is used to check the purity of the gas produced. It enables the cycle to be modified, for example the adsorption phase may be shortened if premature CO2 break-through, due to degradation in the performance of the purification unit as mentioned above, is detected. However, the fact remains that, despite these precautions, after a few years of operation a degradation in the separation performance of the cold box for cryogenically separating syngas is observed.
This lack of performance is attributed to heat exchange deterioration caused by solid deposits on the heat exchanger plates.
Shutting down the unit, to heat (deice) it, enables the problem to be solved but, of course, at a substantial cost if this is not a shut-down programmed in advance. Given the design margins on the heat exchangers taken when designing cold boxes, these effects are felt only after a relatively long period of operation, greater than a year, more generally around 2 to 3 years. This situation makes it impossible to know whether traces of impurities, a priori water and CO2, are being brought by the purified syngas into the cold box after more than one year of service, after several months, or only after a few weeks of operation.
It has been reported that this deterioration stems from chemical reactions between the adsorbent and the adsorbate and/or from reactions between the syngas components, which reactions are promoted by the adsorbent.
The high-temperature reactivity of H2/CO mixtures is in fact well known, but document U.S. Pat. No. 5,897,686 teaches that several reactions occur during the purification repressurization phase, this being a substep of the regeneration. The above document mentions in particular two reactions:                methanation: CO+3H2→CH4+H2O;        the Boudouard reaction: 2CO→C+CO2.        
According to that document, the problem encountered is due to the formation of water in the adsorbent, and the recommended solution is to add, at the top of the adsorber, a bed of 3A molecular sieve which, by not adsorbing CO, prevents in situ formation of said water. That document recommends a regeneration temperature of between 100° C. and 400° C., which corresponds conventionally to a heater skin temperature of from about 150/200° C. to 450/500° C.
Certain chemical reactions may also be catalysed by deposits of secondary constituents on the surface of the adsorbents. Deposits of metals, such as iron, nickel, copper, etc. promote the aforementioned reactions. The origin of some of these metal deposits in due to the decomposition of metal carbonyls formed upstream of the purification.
Progressive poisoning of the adsorbents by traces of impurities, making it difficult or impossible to regenerate said adsorbents, is also a plausible hypothesis knowing the very large number of side-reaction products that may be produced in synthesis reactors, deriving from the coal or natural gas used as raw material, or that may be entrained from upstream prepurification processes, such as methanol scrubbing or amine scrubbing.
Document WO-A-2006/034765 discloses a process for purifying a stream of gas rich in carbon monoxide and hydrogen, in which the gas stream is brought into contact with an adsorption layer containing a silica gel, and the adsorption layer is regenerated with a gas having a temperature of between 70° C. and 150° C., which normally corresponds to a heater skin temperature of around 150° C. to 200/250° C.
The heater skin temperature is defined as the temperature to which the regeneration gas is heated upon passing through the heater, i.e. the temperature of the heat exchange surface in contact with the gas.
Moreover, it is known that, for a given thermal power (Q) expressed for example in Kcal/h, the heat exchange area (S) to be installed is inversely proportional to the temperature difference ΔT between the skin temperature T1 of the heating surface and the temperature T2 of the regeneration gas.
From this, it is readily understood that, to reduce the necessary heat exchange area, and consequently the investment, it is necessary to use a skin temperature T1 as high as possible.
Hence it is common practice to use, in the prior art, a skin temperature T1 such that T1=T2+ΔT, where ΔT≧50° C. and preferably with ΔT around 100° C.
In a refinery or a chemical or petrochemical plant, to heat a fluid to a temperature of 170° C., it is conventional practice to use steam at 250/270° C. or higher.
According to the teaching of document WO-A-2006/034765, the claimed process makes it possible to limit formic acid formation and to extend the lifetime of the adsorbents for said purification.
However, several chemical reactions take place during the heating step, this being a substep of the regeneration.
Despite all these poisoning hypotheses, the main reason for ingress of impurities into to the cold box has yet to be clearly identified.
Hence, one of the problems that arise is how to deliver a syngas of cryogenic quality without having to intervene prematurely on the purification units and/or on the cold box, by providing an effective process intended to purify an H2/CO mixture containing at least one impurity, so as to prevent or minimize undesirable reactions.